IMHO, and in no particular order...
1. Dead Island - Yeah, I was part of the like 75% of the game's playerbase that bought this game primarily because of that really goddamn awesome trailer. It completely failed to live up to expectations on every front. Sluggish controls, poorly balanced character classes, guns were practically worthless, the kick/stomp combo was generally more useful than melee weapons, you spent half the game fighting looters instead of zombies, the voice acting was abysmal, the story was abysmal, glitches galore, ugh... The worst part is that the game had potential. Originally it was supposed to be way more complex, focusing on some fairly serious real-world issues, and the characters were supposed to be much more defined... but most of it got cut out to make a fairly generic game. Lame.
2. Brink - This game sold itself on being a revolutionary shooter. Unique art style, colors other than brown and gray, "tons" of character customization, team objective-based combat where every class was important, etc. The only thing we got that it had promised was an uncanny valley art style and colors, everything else turned out to be a joke... like ridiculously limited character customization that was mostly locked from the get-go and gameplay so completely underwhelming that I couldn't see any reason to play Brink instead of Team Fortress 2.
3. Diablo 3 - Don't get me wrong, I found this to be a fairly well-made game... but that said, it was also an incredible let-down to me. Constant rebalancing, development emphasis being placed on promoting the Real Money Auction House while the gold economy burns to the ground, fairly bland character builds (is there any reason at all to have two of the same class?), and a ludicrously bad story that used pretty much the same environments from Diablo 2. This, like Dead Island, is made even more disappointing when you look back at all of the things that were promised and shown to us over the years while the game was in development that were eventually cut from the final product. Diablo 2 was a game that I played for years and it always managed to hold my attention. I was bored of Diablo 3 within the first month if its lifespan. If there's ever a Diablo 4 (and there will be, because it's a profitable name) I'm likely to be very hesitant to bother with it, assuming I don't just bypass it entirely.
4. Champions Online - I had high hopes that this game would be the new-and-improved City of Heroes. When it finally came out... ugh. The character models were pretty terrible looking, the character customization was far more limited than City of Heroes (granted, it was still a whole hell of a lot better than the vast majority of games), if you changed the shape of your character's body it would stretch and tear many of the costume options (many of them were designed to -only- look right on the basic untouched models), many of the abilities were either completely worthless or incredibly overpowered with little middle ground, there was constant nerfing and "rebalancing" to appease the extremely vocal minority of people playing the game's PvP (meanwhile, many legitimate problems with the game like glitches went completely under the patching radar), the underwater zone had tons of lag issues that went unfixed for close to a month, there were huge XP gaps if you weren't very careful with your questing on launch (with kill XP being so ridiculously low that hitting max level became a massive grind if you ran out of quests), there wasn't very much content (even a casual player could complete it in a week), etc. I could go on, but this is already a wall of text.
5. Duke Nukem Forever - With how long this game spent in development hell, it pretty much needed to cure cancer and end world hunger to not be a disappointment.
